Poly (ADP-rib) polymerase is a chromatin associated enzyme which catalyzes the covalent modification of histones and other nuclear proteins by the transfer of successive units of (ADP-rib) from NAD to these proteins. One important product of this reaction is a crosslinked dimer of histone H1. Our recent major contribution in this area is the in vitro demonstration that long chain poly ADP-ribosylation leads to crosslinking and concomitant complexation of oligonucleosomes. At present it is difficult to propose a specific hypothesis which can be readily tested on the biological function of this complexation reaction, however, we believe that this reaction is linked in some manner to the DNA synthetic/repair processes by holding together domains of chromatin containing singlestrand breaks in a manner which can serve as a recognition site for DNA replicative and repair enzymes. Accordingly, the overall objective of this program (Aims 2 and 3) is first to obtain more precise biochemical and biological characterizations of this reaction. In Aim 2 a novel approach for the study of the nucleosome complexation reaction in living cells is provided as well as the elucidation of the chemical and enzymatic composition of the complex. In addition, the above-mentioned hypothesis will be directly tested by determining whether poly ADP-ribosylation stabilizes adjacent nucleosomes containing internucleosomal DNA with strand breaks. Aime 3 is specifically directed at how the poly (ADP-rib)-H1 dimer may be involved in stabilizing higher-ordered condensation of nucleosomes. The amino acid linkages in the dimer will be determined and the topography of H1-poly (ADP-rib) complexation with chromatin will be studied via chemical crosslinking studies. The experiments in Aim 1 support the above objectives of the program by providing an immunological approach, using antibody to the polymerase, to determine the domains of chromatin undergoing ADP-ribosylation, and to aid in the enrichment of these regions for use in Aims 2 and 3. In addition, we will test whether the polymerase is bound to a subset of DNA sequences in chromatin by the use of this antibody; a second question concerns the localization of this specific nonhistone protein in cells and in regions of polytene chromosomes.